Why do shadows occur?

Hi,
I’m using Blender 2.83LTS.
I subdivided the plane (wall surface).
Then, even though it was a perfect plane, a shadow appeared.


The normals are oriented correctly and the coordinates of the vertices that make up the plane are exactly the same.
Why do shadows occur?

thank you.

You’ve enabled smooth shading, right? Looks like artefacts from that. With perfectly flat objects, you can get away with leaving shading flat, or you can mess around with autosmooth (which you might also have switched on), or you can add a bevel modifier instead which should also help, and will nicely soften edges a little.

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It was caused by automatic smoothness.
Thank you very much.

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Point normals are usually averaged (unless you use flat shading), in your case they point outwards in a diagonal since the bottom face is oriented perfectly downwards (Z axis) and the front face is oriented on the Y axis. You can check that by enabling vertex normals in the overlay popover and moving vertices around.

thank you.

“Smooth shading” is a nice and handy mathematical hack, but it depends entirely on the underlying geometry. Don’t “reflexively turn it on” – sometimes it’s better turned-off.